The field of art to which the invention relates is lifting devices, and more particularly to apparatus for controlling the operation of a lift motor in such a device.
A conventional type of lifting device takes the form of a lift truck including a fork carriage mounted on an extensible upright assembly. The carriage may be raised and lowered by extension and retraction of a hydraulic lift motor which is connected to the carriage by chains reeved within the upright assembly.
Such lift trucks are commonly used in a large variety of applications wherein a load is deposited by an extended upright at a selected elevation above ground level.
It has been common practice heretofore to provide a regulator valve in or associated with the lift motor of such uprights for the purpose of controlling the rate of fluid exhaust from the cylinder of such a motor during lowering or a load as an inverse function of the load on the fork carriage, thus decreasing the lowering speed of the carriage as the load carried thereby is increased. Exemplary valving devices for such a purpose are disclosed in U.S. Pat. Nos. 2,676,573 and 3,016,046.
Both of these patents disclose valve devices in such lift motors for allowing unimpeded pressure fluid flow to the respective lift motor during elevation of a load on the upright, and more or less restricted flow out of the respective lift motor during lowering thereof in order to effect a controlled rate of descent of the load on the upright as a function of the mass of the load carried by the fork carriage. In U.S. Pat. No. 3,016,046 a characteristic inverse relationship between lowering speed and load, which reflects lift motor pressure, is shown in FIG. 3 thereof. Likewise in U.S. Pat. No. 2,676,573 a controlled rate of descent is effected as the lift control valve thereof tends to restrict flow out of the lift motor during lowering of the load as a function of motor pressure, which is desirable. Under a condition of heavy reverse flow surge the latter patent states that the valve device closes off the flow to prevent a sudden drop of the load supported by the upright.
In addition to controlling the rate of descent of such uprights as a function of load, an additional function is desirable, viz., preventing a slack condition of the lifting chains for any reason which might result in a subsequent free drop of the fork and carriage. For example, a slack chain condition can occur if, for any reason, there is an obstruction to lowering of the carriage while an untrained or inattentive operator continues to hold open the directional control valve to attempt to continue to lower the load. Under this condition the weight of the chains, piston head and piston rod continues to exhaust fluid from the lift motor to the reservoir. Then, the sudden release of any obstruction to lowering of the fork carriage and load would permit a free drop thereof until the slack in the chains which connect the lift motor and fork carriage is taken up.
One circumstance which may result in such chain slack is described in the preamble to U.S. Pat. No. 3,438,308. Essentially it is the result of inattention or incompetence of a lift truck operator who continues to exhaust fluid from the lift motor even after the chains begin to go slack when the fork carriage rests on top of a load, such as inside of a pallet and load just deposited at an elevated position. When the truck is then backed off, the fork carriage drops abruptly on moving away from the previously supporting surface and the sudden take up in chain slack may produce needless wear or fatigue in the chains and associated parts.
U.S. Pat. No. 3,438,308 discloses a valve device which is constructed to admit a flow of pressure fluid to a lift motor during elevation of a lift truck upright, to exhaust pressure fluid from the lift motor during normal operation thereof, and to prevent the continued exhaust of fluid from the lift motor upon a predetermined decrease in the fluid pressure therein for preventing a slack chain condition.